1. Field of the Invention
The present invention relates to an apparatus for controlling an electric motor and more particularly to an apparatus for controlling an electric motor adapted to achieve improvement of a power factor of a three phase alternating current power source by the use of a diode converter and an inverter of a pulse width modulation type.
2. Description of the Prior Art
It is generally known that an alternating current (hereinafter to be briefly called AC) electric motor may be driven controlled by a controlling apparatus which employs a pulse width modulation type inverter (hereinafter to be briefly called a PWM inverter). For example, since a voltage type PWM inverter itself has a function of voltage regulation, the magnitude of the output voltage can be varied by firing control of the inverter. Also, since the PWM inverter can control its output voltage instantaneously, its output current can be controlled to have a sinusoidal waveform.
As an example of the conventional apparatus for controlling an electric motor, there is one invented by Okuyama et al. and for which an application for a patent was filed in the Patent Office of Japan by Hitachi, Ltd. on Jan. 10, 1979. This invention was also the subject of an application for U.S. patent on Jan. 7, 1980 under the title of "APPARATUS FOR CONTROLLING AC MOTOR", with the right of priority claimed, the patent therefor being granted as U.S. Pat. No. 4,328,454. The prior art controlling apparatus is connected with a three-phase AC power source at one end and with a three-phase AC electric motor at the other end as shown in a schematic circuit diagram of FIG. 1.
Referring to the figure, reference numeral 1 denotes a three-phase AC power source, 2 denotes a first power converting device of a pulse wave modulation type, 3 denotes a three-phase AC electric motor such as an induction motor, 4 denotes a second power converting device of a pulse width modulation type, 5 denotes AC reactors whose group of terminals on one side are connected with the three-phase AC power source 1; the group of terminals of the AC reactors 5 on the other side are connected with the terminals on the AC side of the first power converting device 2. Reference numeral 6 denotes a DC smoothing capacitor, and this DC smoothing capacitor 6 is connected with terminals on the DC side of the first power converting device 2. This DC smoothing capacitor 6 is further connected to terminals on the DC side of the second power converting device 4, and terminals on the AC side of the second power converting device 4 are connected to the three-phase AC electric motor 3. Reference numeral 7 denotes a DC voltage instructing circuit outputting an instruction value of a reference voltage, 8 denotes a DC voltage detector for detecting the voltage across the above mentioned DC smoothing capacitor 6, 9 denotes a deviation amplifier for calculating deviation of the output of the DC voltage detector 8 from the output of the DC voltage instructing circuit 7 and amplifying and outputting the deviation, 10 denotes an insulated transformer for detecting the three-phase voltage of the three-phase AC power source 1, 11 denotes a current detector for detecting the three-phase AC current in the first power converting device 2, and 12 denotes an arithmetic unit making a calculation based on the output of the deviation amplifier 9, the output of the insulated transformer 10, and the output of the current detector 11 thereby to output an instruction signal for the three-phase voltage on the AC side of the first power converting device 2. Reference numeral 13 denotes a triangular wave generator, 14 denotes a comparator for comparing the output of the arithmetic unit 12 with the output of the triangular wave generator 13 thereby to output an ON/OFF signal for the switching element in the first power converting device 2, 15 denotes a gate amplifier circuit for amplifying the ON/OFF signal output from the comparator 14 thereby to ON/OFF control the gate turn off (GTO) thyristors as the switching element in the first power converting device 2.
On the other hand, 16 denotes a frequency instructing circuit for specifying the output frequency and voltage of the second power converting device 4, 17 denotes an oscillator for generating a three-phase sinusoidal wave signal of a variable frequency, 18 denotes a multiplier for multiplying together the output signals from the frequency instructing circuit 16 and the oscillator 17, 19 denotes a triangular wave generator, 20 denotes a comparator for comparing each output of the multiplier 18 with the output of the triangular wave generator 19 thereby to output an ON/OFF signal for the switching element in the second power converting device 4, and 21 denotes a gate amplifier circuit for amplifying the ON/OFF signal output from the comparator 20 thereby to ON/OFF control the switching element in the second power converting device 4.
The AC reactors 5, first power converting device 2, DC voltage instructing circuit 7, DC voltage detector 8, deviation amplifier 9, insulated transformer 10, current detector 11, arithmetic unit 12, triangular wave generator 13, comparator 14, and the gate amplifier circuit 15 put together will hereinafter be referred to as the power converting apparatus A.
Now, the operation will be described. The first power converting device 2 has a function of varying the relationship between magnitudes of the voltage on the AC side and the voltage on the DC side by the ON/OFF control of the switching element therein.
Firstly, the operation by which the above mentioned function can be obtained will be described. The DC voltage instruction V.sub.d.sup.* delivered from the DC voltage instructing circuit 7 and the voltage V.sub.d on the DC smoothing capacitor 6 are input to the deviation amplifier 9 and the difference therebetween is amplified and output. This output becomes the instruction i.sub.d.sup.* for the current i.sub.d on the DC side of the first power converting device 2. By making the direction of the current i.sub.d positive when it flows from the first power converting device 2 to the DC smoothing capacitor 6, the instruction i.sub.d.sup.* is output to be positive when V.sub.d.sup.* &gt;V.sub.d and negative when V.sub.d.sup.* &lt;V.sub.d.
The arithmetic unit 12 receives the supply of i.sub.d.sup.*, the output of the insulated transformer 10, and the output of the current detector 11 and calculates and delivers instructions V.sub.u.sup.*, V.sub.v.sup.*, and V.sub.w.sup.* for the voltages on the AC side of the first power converting device 2. There being various ways to calculate and output those V.sub.u.sup.*, V.sub.v.sup.*, and V.sub.w.sup.*, one of them will be mentioned below.
The value i.sub.d.sup.* is multiplied by each of KV.sub.R, KV.sub.S, and KV.sub.T which are proportional to the three-phase voltages V.sub.R, V.sub.S, and V.sub.T of the three-phase AC power source 1 output from the insulated transformer 10. As a result, KV.sub.R .multidot.i.sub.d.sup.*, KV.sub.S .multidot.i.sub.d.sup.*, and KV.sub.T .multidot.i.sub.d.sup.* are obtained. These correspond to the AC current instructions i.sub.u.sup.*, i.sub.v.sup.*, and u.sub.w.sup.* for the first power converting device 2. If the actual AC currents i.sub.u, i.sub.v, and i.sub.w coincide with i.sub.u.sup.*, i.sub.v.sup.*, and i.sub.w.sup.*, then these are in phase with or out of phase of V.sub.R, V.sub.S, and V.sub.T and hence the power factor of the power source becomes .+-.1. The values i.sub.u.sup.*, i.sub.v.sup.*, i.sub.w.sup.*, i.sub.u, I.sub.v, and i.sub.w are made positive when in the direction flowing from the three-phase AC power source 1 to the first power converting device 2. The values V.sub.u.sup.*, V.sub.v.sup.*, and V.sub.w.sup.* are obtained by amplifying (i.sub.u.sup.* -i.sub.u), (i.sub.v.sup.* -i.sub.v), and (i.sub.w.sup.* -i.sub.w), respectively.
The mentioned values V.sub.u.sup.*, V.sub.v.sup.*, and V.sub.w.sup.* are compared in the comparator 14 with a triangular wave generated by the triangular wave generator 13. The output from the comparator 14 is amplified in the gate amplifier circuit 15, and thereby, the switching element in the first power converting device 2 is ON/OFF controlled, when the phase voltages V.sub.u, V.sub.v, and V.sub.w on the AC side of the first power converting device 2 take pulse waveforms, but their fundamental waves agree with V.sub.u.sup.*, V.sub.v.sup.*, and V.sub.w.sup.*.
FIG. 2(a) shows a vector diagram of V.sub.R, V.sub.u, and i.sub.u when the when the power is supplied from the AC side to the DC side of the first power converting device 2 and FIG. 2(b) shows the same when the power is conversely supplied from the DC side to the AC side. It is adapted such that power of a variable voltage and variable frequency is supplied to the three-phase AC electric motor 3 by ON/OFF controlling of the switching element in the second power converting device 4 made by the gate amplifier circuit 21.
Since the apparatus for controlling an electric motor of the prior art was structured as described above, there was a problem that, when power conversion from a three-phase AC power source to a DC power supply was to be made, the first power converting device (converter) had to be provided with a converting device similar to that for driving the electric motor, i.e., the second power converting device (inverter), and besides, some detectors were required for detecting such values as the voltage phase and supply current on the AC power source side, and hence, the system as a whole became rather larger in size.
Incidentally, as the above described inverter, a general purpose inverter can be employed such as, for example, that described in the disclosure, "GENERAL PURPOSE VARIABLE FREQUENCY INVERTER USING INTEGRATED POWER MODULE AND LSI" submitted to IEEE by MITSUBISHI DENKI KABUSHIKI KAISHA (Mitsubishi Electric Corporation).